1. Field of the Invention
The invention relates to an ink jet print head with mutually parallel ducts formed inside a substrate and separated by partition walls. The ducts are provided with a cover plate and one outlet opening on each of their ends. One thermal or piezoelectric element is associated with each duct. Upon excitation and with ink fluid disposed inside the duct, the element effects an expulsion of a drop of ink from the outlet opening. The invention further relates to a method of producing such an ink jet print head.
2. Description of the Related Art
Ink jet print heads are widely used in ink jet printers. The ink jet print head usually operates by the known drop on demand or DOD method, described for instance in German Patent DE 30 12 698 C2. There, to create a dot on a medium to be imprinted, such as paper, a drop of ink is expelled from a duct of the ink jet print head as soon as a thermal or piezoelectric element associated with the duct is triggered with a suitable current pulse from a driver circuit. The excitation occurs as the result of a current pulse 2 .mu.s to 10 .mu.s in duration, for instance, thus releasing thermal energy of approximately 15 to 50 microjoules. This heating leads to local evaporation of the ink fluid (bubble formation). The column of fluid is positively displaced from the corresponding duct outlet opening but without initially tearing. Once the current pulse ends, the bubble collapses above the thermal element. As a consequence, some of the fluid column is drawn back in. A drop of ink separates from the column outside the duct outlet openings and moves onward due to the conservation of momentum. These drops of ink create a black printed dot, in the case of black ink, on the paper. The typical emission frequency is approximately 5 kHz.
To create a character, such as a letter, the thermal or piezoelectric elements of the parallel ducts must be suitably supplied with current pulses by the driver circuit in such a way that the dots required for these letters become visible on the paper as a result of the impact of corresponding drops of ink.
Because of the very small duct diameter and close matrix spacings between the ducts (or jets), processing methods known from semiconductor technology are employed to create ink jet print heads. Examples of such processing methods are described in European Patent Disclosures EP 0 359 417 A2 and EP 0 434 946 A2, and in IEEE Transactions on Electron Devices, Vol. 26, 1979, p. 1918. In contrast to the production of integrated semiconductor circuits, which are formed on a single substrate, the prior art methods for producing ink jet print heads require at least two different substrates. On one substrate, partitions between ducts are formed, and these are closed by a separately produced cover plate made of a second substrate.
In the prior art methods, heating resistors can be disposed on or in the duct for thermal excitation. Often the ducts are formed by orientation-dependent etching in a silicon substrate. The heating resistors can be secured to the ducts by bonding. A glass plate, for instance, may be used as the cover plate. The glass plate is mounted on the duct plate, and hence in the first substrate, by anodic bonding.
As disclosed by the European document EP 0 443 722 A2, the ducts of the ink jet print head can also be formed by adjusting a cover plate, provided with partitions, onto a first substrate that is provided with heating resistors. Instead of the cover plate provided with partitions, a flat cover plate can also be glued to the first substrate, if the aforementioned ducts have already been machined into the first substrate, in the form of duct bottoms and two duct side walls each. The glued-on cover plate then forms the top of the duct for these ducts.
A problem associated with these prior art methods for producing integratable ink jet print heads is the absolute necessity of two substrates that must be joined to one another. This requires complicated adjustment, and the fine conduits must be protected against contamination while the two substrates are being glued together, which means additional effort and expense.